This invention relates to an abnormality detecting device for internal combustion engines, and more specifically to an abnormality detecting device for detecting abnormality in the air-fuel ratio sensors disposed on the upstream and downstream sides of the catalytic converter.
Generally, in the exhaust system of an internal combustion engine, there are provided a catalytic converter for controlling the discharge of the emission and an O.sub.2 sensor for monitoring the oxygen density of the emission. In recent years, the emission regulations have been reinforced, as the result of which it has been required that the component parts such as the catalytic converter and the O.sub.2 sensor should be checked for abnormality.
Japanese Utility Model Application Laid-Open No. Hei-3-87949 discloses a device for detecting abnormality of the O.sub.2 sensors. In this abnormality detecting device, two O.sub.2 sensors are provided, one on the upstream side and the other on the downstream side of the catalytic converter installed in the exhaust system. When the catalytic converter is in the inactive state, the output response time of the downstream side O.sub.2 sensor is detected, and from this output response time, abnormality of the O.sub.2 sensor, its deterioration, for example, is determined.
However, generally speaking, if the catalytic converter is in the inactive state, since the temperature of this converter and its surroundings is not raised, so that the O.sub.2 sensor is often in the inactive state, too. And, when in the inactive state, even if its function is normal, the O.sub.2 sensor is sometimes erroneously determined as abnormal by the abnormality detecting device. For this reason, the conventional abnormality detecting device has a problem in terms of accuracy of abnormality detection.